messaging3: Fix messaging_read_send/recv

messaging3: Fix messaging_read_send/recv

messaging_read_send/recv was okay for just one handler in the queue. For
multiple handlers it was pretty broken.

A handler that deletes itself as part of the callback (pretty typical use
case...) drops the message for a subsequent handler that responds to the same
message type. In messaging_dispatch_rec we walk the array, however
messaging_read_cleanup has already changed the array. tevent_req_defer_callback
does not help here: It only defers the callback, it does not defer the cleanup
function.

This also happens when a callback deletes a different handler

A handler that re-installs itself in the callback might get a message twice.

This patch changes the code such that only messaging_dispatch_rec adds records
to msg_ctx->waiters, new waiters are put into a staging area first
(msg_ctx->new_waiters). Also messaging_read_cleanup does not move anything
around in msg_ctx->waiters, it only nulls out itself. messaging_dispatch_rec is
changed to cope with this.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>